How do I create this arch in the middle of a cube?
You should use the Array modifier with an Object Offset:

Create a 8 vertex circle.



Only keep 1/8, create an empty at the same point as its origin.



Give your circle an Array modifier, Count &gt; 8, deactivate Relative Offset, activate Object Offset, choose the empty as object, rotate the empty 45° on the Z axis.



Model the object.



Extrude it down, flatten the bottom with SZ0.